The present invention relates generally to serial communications and, more particularly, to high speed low power asynchronous serial communications.
High speed serial communication devices transform wide bit-width, single-ended signal buses and compress them into a few, typically one, differential signal that switches at a much higher frequency rate than the wide single-ended data bus. High speed serial communication devices enable the movement of a large amount of data point-to-point while reducing the complexity, cost, power, and board space usage associated with having to implement wide parallel data buses.
Several high speed serial communication interface standards are particularly developed for mobile applications to obtain low pin count combined with improved power efficiency.
Unlike traditional high speed serial links, serial communication interface standards such as MIPI (Mobile Industry Processor Interface) M-PHY use a BURST mode operation for improved power efficiency. The burst mode of operation allows the links to be turned off when not required which improves the power efficiency. This presents a problem while re-starting the high speed serial link when required. To enable this burst mode operation, the M-PHY standard uses a series of specific symbols called PREPARE prior to starting the transmission of a new payload data. The receiver side is expected to detect the start of a transmission burst based on sequence of the PREPARE symbols. To ease the implementation and to ensure high degree of detectability, a PREPARE symbol is defined to be sequence of 10 ones.
The start of a burst may be defined by some minimum number of PREPARE symbols. For example, the M-PHY standard specified minimum of four PREPARE symbols. Note that the high speed serial links use some form of line coding to ensure that there are enough transitions in the transmitted data to enable the receiver to recover the clock and data. For example, the M-PHY standard uses the 8b10b coding to ensure that there is sufficient density of transitions and the DC balance is maintained. The PREPARE symbol is not considered to be a part of the normal payload data and therefore it does not follow the 8b10b coding rules.
At the receiver side, typically the incoming differential signal is first amplified, equalized, time aligned and sliced to extract the decoded serial bits. These serial bits are then further processed to detect the sequence of PREPARE symbols.
Conventional methods detect the start of the burst by using different methods but generally involve counting the number of consecutive ones received in the incoming data. This method is simple and generally effective. However, under poor signal conditions the incoming signal may be corrupted by a variety of noise sources which can cause random errors in the decoding of the incoming data. These random errors may prevent the receiver from detecting the start of a burst. This in turn leads to the loss of information contained in the entire burst. This can lead to retransmissions at higher protocol layer, which may results in latencies that may not be acceptable in some applications. Therefore, it is essential to detect the start of a burst by detecting the PREPARE symbols with very high degree success.
Conventional methods for PREPARE symbols detection fail to achieve the required degree of reliability or they may require the transmitter to send a longer sequence of PREPARE symbols. Sending longer number of PREPARE symbols reduces the bandwidth utilization of the high speed serial link which is not desirable.